The configuration of a sheet feeder incorporated into a conventional image reading apparatus will be explained with reference to FIGS. 13 to 17, but the same reference numerals as in embodiments to be described later denote the same parts, and an explanation thereof will be omitted.
FIGS. 13 to 16 show the orientation of the leading edges of stacked documents D when a conventional sheet feeder 102 feeds the documents. When feed is started from a document sheet set state shown in FIG. 13, the leading edges of the stacked documents are obliquely misaligned along an inclined surface 2b of a lower guide unit 2 as shown in FIG. 14, and the number of stacked documents D decreases when feed is continued as shown in FIG. 15. When feed is performed in this state shown in FIG. 15, the uppermost document sheet to be fed is given no slack and pulled between a pickup roller 8 and feed roller 9 as shown in FIG. 16, because the rotational speed of the feed roller 9 is higher than that of the pickup roller 8. Assuming that a triangular region connecting a contact point p1 between the pickup roller 8 and the uppermost document sheet, a contact point p2 between the feed roller 9 and a separation roller 4, and an intersection p3 between a document sheet stacker 2a and the inclined surface 2b is a warp space, a portion of the uppermost document sheet, which is close to this warp space, moves in the direction of an arrow e.
In the process from FIG. 15 to FIG. 16, when the portion of the uppermost document sheet, which is close to the warp space, is displaced in the direction of the arrow e, the leading edge of the second document sheet is stopped by the separation roller 4, so a portion of the second document sheet, which is close to the warp space, is in tight contact with the uppermost document sheet. Accordingly, the second document sheet is displaced in the direction of the arrow e following the movement of the uppermost document sheet. Referring to FIG. 16, a length L2 of the second document sheet between the points p1 and p2 is larger than a length L1 of the uppermost document sheet between the points p1 and p2. When the second document sheet moves in the direction of the arrow e, therefore, the second document sheet readily slackens in the warp space as shown in FIG. 17. Also, since the frictional force between the uppermost document sheet and second document sheet in contact with each other at the point P1 is large, there is the possibility that the second document sheet moves following the feed of the uppermost document sheet and warps in the warp space. In particular, a thin document sheet such as paper or a film readily warps. If this warp of the document sheet increases in the warp space, the document sheet bends and becomes wrinkling, and cannot smoothly enter the nip between the feed roller 9 and separation roller 4 any longer. This causes a feed defect.
As a related art, Patent Document 1 describes a technique by which when the number of stacked documents on a feed tray becomes smaller than a predetermined reference number, the rotation of an eccentric cam driven by a cam driving motor raises a lifting member, the lifting member lifts a document sheet picked up by a pickup roller, thereby making the entrance angle of the document sheet almost horizontal with respect to a feed nip portion regardless of the number of stacked documents on the feed tray.